Liposomes are vesicles with an aqueous interior enclosed by one or more phospholipid bilayers. Such vesicles have demonstrated utility as vehicles for delivering therapeutic and/or diagnostic agents to target tissues or organs. Liposomes that have been introduced into the circulatory system are removed therefrom by macrophages, particularly those of the spleen and liver that are in direct contact with the circulatory system, in a process referred to as phagocytosis. Phagocytosis is essential for immune surveillance and response. Although nearly all microorganisms phagocytosed by macrophages are degraded enzymatically following fusion of the phagosomes (containing the microorganisms) with host lysosomes, some pathogens survive phagocytosis by interrupting the natural progression of the phagosome to the host lysosomes. Indeed, some of the most significant pathogens, e.g., the agents of typhoid fever, Legionnaire's disease and tuberculosis, evade macrophage defenses by taking up residence within the phagosomes of macrophages.
Several antibiotics have been encapsulated into liposomes for the purpose of treating intracellular parasites in macrophages. For example, amphotericin B, encapsulated inside liposomes, reportedly has been used to treat leishmaniasis, histoplasmosis and candidasis, (Alving, C. et al., Proc. Natl. Acad. Sci. USA 75:2959-2963 (1978); Szoka, F. et al., Antimicrobial Agents and Chemotherapy 31:421-429 (1987)). Liposomes containing amikacin reportedly exhibit improved efficacy in the treatment of Mycobacterium avium complex, a significant pathogen of AIDS patients (Duzgunes, N. et al., Antimicrobial Agents and Chemotherapy 32:1404-1411 (1988)). Liposomes containing gentamicin or ampicillin reportedly are slightly more effective than free drug when tested in a mouse model of Salmonellosis (Fattal, E. et al., Antimicrobial Agents and Chemotherapy 35:770-772 (1991); Swenson, C. et al., Antimicrobial Agents and Chemotherapy 34:235-240 (1990)).
Despite these reported successes, one problem inherent to conventional liposome-based drug delivery is that conventional liposomes cannot deliver drugs or other agents to the cytoplasm of the target cell. This is because once the liposome has been internalized within a phagosome of the target cell, the phagosomal membrane acts as a barrier and blocks passage of the liposome-entrapped drug or agent to the cytoplasm. Accordingly, there currently is no effective method to deliver a membrane impermeant drug to the cytoplasm of a cell to enable the drug to interact with its intended target.
Moreover, conventional liposome-delivered agents often are degraded before reaching their intracellular target because liposomes ingested by macrophage phagocytosis are delivered into lysosomes in the same manner as phagocytosed particles or microorganisms. In addition, not every liposome-encapsulated drug that is phagocytosed by a pathogen-infected macrophage will be delivered to every phagosome that contains a pathogenic microorganism, i.e., some pathogens will avoid exposure to the drug simply because the liposome is not delivered to the phagosome in which the pathogen resides. The intracellular pathogen or other target of drug therapy may avoid the drug because it resides within the cell but outside the phagosome.
In view of the foregoing, there is still a need for improved liposomes for facilitating the delivery of extracellular agents, such as antibiotics, into target cells to deliver the extracellular agent to an intracellular (cytoplasmic) target. Such improved liposomes would deliver the extracellular agent to the intracellular target structure with reduced lysosomal enzyme catalyzed agent degradation in comparison with the degradation observed in connection with drug delivery using the prior art liposomes. In addition, the improved liposomes would permit the delivery of a higher effective intracellular concentration of the extracellular agent to the intracellular target and would permit the targeting of extracellular agents to particular structures within the cytoplasm of the target cell.